This invention relates to gas monitors.
Investigation of the levels of various pollutants in ambient air requires a reliable means of sampling the air and quantitatively determining the concentration of the pollutant of interest. Although pump-driven air sampling techniques are frequently employed, the passive type of sampler, which relies upon molecular diffusion of gases, has proven to be comparatively simple, lightweight, easy to use, inexpensive and nonhazardous, and therefore more suitable for certain applications. Multiple passive samplers can be used to make simultaneous multi-point measurements or repeated measurements over time at a single location, and are therefore an important tool for large-scale air pollution studies involving wide temporal or spatial variables. Also, the passive design may be incorporated into a device intended to monitor an individual's personal exposure to a given pollutant over a period of time.
Generally, the passive gas sampler consists of a collection apparatus containing a collecting medium. The apparatus is typically a container with an orifice at one end to permit ambient gases to diffuse in and thereby contact the collecting medium. Insertion of a tube through the orifice and part way into the container permits the diffusion rate, which is proportional to the cross-sectional area of the diffusion path and inversely proportional to its length, to be reduced without reducing the surface area of collecting medium used (Palmes et al., Am. Ind. Hyg. Ass. J. 34:78-81, 1973; Palmes et al., Anal. Chem. 51:2400-2401, 1979).
The collecting medium may be in either liquid or solid form. The liquid type, which may be coated onto a mechanical support, typically absorbs the pollutant gas by chemically reacting with it. An example of a liquid reagent is triethanolamine, widely used to collect NO.sub.2 (e.g., Palmes et al., Am. Ind. Hyg. Ass. J. 37:570-577, 1976; Yanagisawa et al., Environ. Int. 8:235-242, 1982).
The solid type of collecting medium commonly traps the molecules of pollutant gas by adsorption. Activated charcoal, frequently used in gas monitors to collect benzene and other kinds of organic vapors, is one such solid adsorbent. Adsorbed gas may be recovered from the solid adsorbent by extraction with an appropriate solvent, by reducing atmospheric pressure on the adsorbent, or by heating the adsorbent until the gas is released ("thermal desorption").
Passive samplers specifically designed to determine the level of carbon monoxide in ambient air have included two kinds of direct-read dosimeters: a degree-of-color-change device, in which carbon monoxide permeating a thin bed of colorimetric granular indicator changes the color of the indicator to a degree dependant upon the dose of carbon monoxide adsorbed (U.S. Pat. No. 3,507,623); and a length-of-stain device, in which carbon monoxide diffusing lengthwise through a bed of inert carrier impregnated with an indicator generates a color change along the diffusion path, the length of which is proportional to the concentration of the gas and the sample time (McConnaughey et al., Am. Ind. Hyg. Assoc. J. 46:357-362, 1985). Indicators which change color when exposed to carbon monoxide have typically utilized either palladium(II) or silver compounds (Lambert et al., Environ. Sci. Technol. 21:500-503, 1987).